The present invention relates to nuclear power plants and more particularly to monitoring systems for snubbers used to support plant piping and equipment to withstand seismic and vibratory loading while permitting thermal expansion and contraction.
Power plants normally operate through heatup and cooldown cycles. As temperature changes occur, piping systems and equipment move as a result of thermal expansion and contraction. In the basic plant design, support systems need to be designed to allow thermal movement and thereby avoid overstressing the piping and equipment. At the same time, the piping and equipment needs to be supported rigidly to withstand seismic and vibratory loading such as water hammer loading.
Resolution of these seeming contradictory requirements on the support systems design is achieved through the use of special support members. These devices are known as snubbers and are used extensively throughout a nuclear power plant to provide piping and equipment support that permits the supported piping or equipment to move under thermal loading yet hold the piping or equipment rigid during seismic or other vibratory events.
A typical snubber can be mechanical or hydraulic in design. Most snubbers are shaped like automobile shock absorbers, but they vary greatly in size, weight, and load capacity. With proper functioning, a snubber extends or contracts slowly along its axis under thermal loading. Under sudden seismic or vibratory type accelerations, the snubber becomes rigid or "locks-up".
To reduce or avoid snubber failures and thereby enhance plant safety, The Nuclear Regulatory Commission requires electric utilities to perform snubber operability testing. Typically, such testing during plant shutdowns has involved selecting a predetermined fraction of the plant snubbers for four failure modes. These modes are:
1. Failure to lock up under seismic and vibratory loading. PA1 2. Snubber hydraulic system leaks that allow snubber contraction or expansion in exces of prescribed rates under seismic and vibratory loadings. PA1 3. Inadvertent lock-up that prevents thermal movement of supported piping or equipment which could thus undergo high thermal stress and fatigue. PA1 4. Excessive drag force on expansion and contraction of the snubber thereby restricting thermal movement and increasing thermal stress in supported piping or equipment.
If snubber failures are found in the testing program, statistical sampling methodology is used to increase the sample size. With some snubbers being rather massive and located in highly radioactive areas, and especially with larger test sample sizes, snubber testing programs can be very costly. More importantly, snubber testing programs provide limited enhancement of plant safety since only sampled snubbers are tested and since only one-shot testing is performed during shutdown and not continuously during plant operation.
In U.S. Pat. No. 4, 550,589, entitled SYSTEM FOR MONITORING SNUBBER LOADS and issued to W.S Wright on May 2, 1984, a monitor system is disclosed that more continuous and more extensive snubber monitoring. However, that system generally employs a threshold comparator circuit located near each snubber to store a predetermined threshold value of snubber displacement. The outputs of the comparators are radio linked to a data recorder outside the plant containment. All of the snubbers may be monitored but the system is limited to a single excessive displacement value comparison test. While excessive snubber displacements can thus be detected continuously, insufficient displacements cannot be detected at all. Further, different plant operating modes create varying operating conditions which result in different excessive displacement criteria for the different modes and such differences cannot be accommodated by the prior art system. Accordingly, the prior art is limited in the flexibility and the completeness with which snubber monitoring is achieved.
The present invention is directed to a improved snubber monitoring system characterized with more complete and more flexible snubber monitoring and thereby enabling costly snubber testing programs to be reduced in scope significantly or totally and providing improved nuclear power plant safety.